Re: [dpdk-dev] [PATCH v3] net/af_packet: fix ignoring full ring on tx

The poll call can return POLLERR which is ignored, or it can return
POLLOUT, even if there are no free frames in the mmap-ed area.

We can account for both of these cases by re-checking if the next
frame is empty before writing into it.

We have attempted to reproduce this issue with pktgen-dpdk, using the
following configuration.

pktgen -l 1-4 -n 4 --proc-type=primary --no-pci --no-telemetry \
     --no-huge -m 512 \
     --vdev=net_af_packet0,iface=eth1,blocksz=16384,framesz=8192, \
     framecnt=2048,qpairs=1,qdisc_bypass=0 \
     -- \
     -P \
     -T \
     -m "3.0" \
     -f themes/black-yellow.theme

We configure a low tx rate (~ 335 packets / second) and a small
packet size, of about 300 Bytes from the pktgen CLI.

set 0 size 300
set 0 rate 0.008
set 0 burst 1
start 0

After bringing the interface down, and up again, we seem to arrive
in a state in which the tx rate is inconsistent, and does not recover.

ifconfig eth1 down; sleep 7; ifconfig eth1 up

[1] http://code.dpdk.org/pktgen-dpdk/pktgen-20.11.2/source/INSTALL.md

Signed-off-by: Mihai Pogonaru <redacted>
Signed-off-by: Tudor Cornea <redacted>

---
v2:
* Added check for POLLERR
* Used tx_ring_status_available() for checking TP_STATUS_AVAILABLE
---
  drivers/net/af_packet/rte_eth_af_packet.c | 26 ++++++++++++++++++++++++--
  1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/drivers/net/af_packet/rte_eth_af_packet.c b/drivers/net/af_packet/rte_eth_af_packet.c
index 559f5a0..d3d3104 100644
--- a/drivers/net/af_packet/rte_eth_af_packet.c
+++ b/drivers/net/af_packet/rte_eth_af_packet.c

@@ -237,8 +237,30 @@ eth_af_packet_tx(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
  		}
  
  		/* point at the next incoming frame */
-		if (!tx_ring_status_available(ppd->tp_status) &&
-		    poll(&pfd, 1, -1) < 0)
+		if (!tx_ring_status_available(ppd->tp_status)) {
+			if (poll(&pfd, 1, -1) < 0)
+				break;
+			if (pfd.revents & POLLERR)
+				break;
+		}
+
+		/*
+		 * Poll can return POLLERR if the interface is down
+		 *

+		 * It will almost always return POLLOUT, even if there
+		 * are no extra buffers available
+		 *
+		 * This happens, because packet_poll() calls datagram_poll()
+		 * which checks the space left in the socket buffer and,
+		 * in the case of packet_mmap, the default socket buffer length
+		 * doesn't match the requested size for the tx_ring.
+		 * As such, there is almost always space left in socket buffer,
+		 * which doesn't seem to be correlated to the requested size
+		 * for the tx_ring in packet_mmap.
+		 *
+		 * This results in poll() returning POLLOUT.
+		 */
+		if (!tx_ring_status_available(ppd->tp_status))
  			break;
  
  		/* copy the tx frame data */